The present invention relates to an expansion device for an air conditioner, in particular for an air conditioner of a motor vehicle. In addition, the present invention relates to this type of air conditioner.
In the case of air conditioners, particularly compression air conditioners, a compression of a refrigerant is first performed via a compressor, the so-called air conditioner compressor or a simple compressor, in order to increase the internal energy of the refrigerant. The compressed refrigerant is then cooled in a condenser or liquefier via heat exchange. The refrigerant liquefies in the process due to the heat emission that goes hand in hand with the heat exchange. Then the liquefied refrigerant is guided through a throttling point of the air conditioner, whereby this throttling point features an expansion device. The refrigerant expands when it passes through the expansion device. As a rule, an evaporator is subsequently connected to the expansion device and it is operated as a heat exchanger in order to transfer the cold that is being released onto another medium. This other medium can, e.g., be air, which is supplied to a vehicle heating or cooling system.
In order to improve the thermal efficiency of an air conditioner, it is known from the prior air to bring the refrigerant located in the condenser to a super-critical state on the high-pressure side of the closed refrigerant cycle in order to thereby improve the heat exchange performance of the air conditioner. The condenser is then designated as a gas cooler since condensation no longer occurs, but only a cooling of the super-critical gas. In addition, super-critically operated compressor refrigerating machines make it possible for a refrigerant (e.g., carbon dioxide) with a smaller compression ratio to be used.
Due to the control of throttling on the expansion device of the refrigerant cycle, the pressure on the high-pressure side of the thermodynamic cycle process can be controlled or regulated so that the specific cooling performance of the air conditioner can be thereby varied. Thus, it is possible to obtain a maximum effective performance number by setting the pressure of the high-pressure side of the thermodynamic refrigerant cycle in an adapted manner as a function of the temperature at the gas cooler outlet or the ambient temperature.
Particularly in the case of refrigerating systems with refrigerants, which emit heat in the super-critical range due to their thermodynamic properties, the pressure during heat emission can be set independent of the temperature. Using controllable and/or regulatable expansion valves as expansion devices is known in order to be able to advantageously utilize this additional degree of freedom (as opposed to refrigerants with heat emission in the moist vapor range). Currently, both purely mechanical as well as electrically driven expansion valves are being developed, whereby each of these developments of a new expansion devices for a refrigerant cycle of an air conditioner is associated with high costs.
In the case of air conditioners with condensation, overheating of the refrigerant from the evaporator can be controlled via controlling the throttling at the expansion device of the refrigerant cycle in such a way that the cold performance is optimal.
Expansion valves are designed predominantly as valves with a ball seat and an electromagnetic drive. These valves are disadvantageous because of extreme noise development in certain operating states. Particularly in the case of low volume flows of the refrigerant and therefore in the case of valves that are almost closed, it is possible for the valve ball to impact the seat, thereby leading to the distinct development of noise. For this reason, regulating pressure via the expansion device with valves of the ball-seat type is problematic since these go hand in hand with the previously described development of noise.
Valves with sliding seats that have low-noise operation are also known from the field of fuel injection. The disadvantage of this type of valve is the poor imperviousness of the completely closed valve. Because of the function-induced gap between the slider (piston) and the valve seat (guide), when there is a great pressure difference such as that that inevitably occurs in air conditioners particularly with the refrigerant CO2, there is a refrigerant volume flow that is too great for operating a refrigerating system. In particular, because of this leakage, sufficiently great pressure cannot be built up at low pressures in order to be able to operate the air conditioner in a favorable state (efficiency/performance). For this reason, sliding seat valves appear to be unusable as an expansion device for an air conditioning cycle.
The objective on which the invention is based is providing an expansion valve for a refrigerant cycle, which guarantees good controllability with low-noise operation in all operating states of the air conditioner.
In addition, the invention is based on realizing an air conditioner, in particular an air conditioner for a motor vehicle, which makes an improvement in the thermal efficiency of the air conditioner possible through the use of the controllable and/or regulatable expansion valves.